In order to convert video signals of interlaced scanning into video signals of progressive scanning or convert video signals of progressive scanning into enlarged or reduced video signals, interpolation circuits for performing scanning lines interpolation processing are used. In such interpolation circuits, the value of a pixel to be produced by the interpolation processing, that is, a pixel to be interpolated (hereinafter referred to as an interpolation pixel) is calculated on the basis of the values of pixels around the interpolation pixel. In this case, the value of the interpolation pixel is calculated using the pixels in the direction in which there is a high correlation out of the pixels around the interpolation pixel.
In a picture having a diagonal edge or a picture having a thin diagonal line, for example, the value of an interpolation pixel is calculated using pixels in diagonal directions, centered at the interpolation pixel. Therefore, correlation judgment circuits for judging the direction in which there is a high correlation in a picture represented by a video signal are used.
In conventional correlation judgment circuits, a difference value between two pixels in each of a vertical direction and a diagonal direction, centered at an interpolation pixel is detected, and the direction in which there is a high correlation is judged on the basis of the difference value. In such a method using a difference value between two pixels, however, erroneous judgment may, in some cases, be made. When interpolation processing is performed in a picture having a diagonal edge or a picture having a thin diagonal line, a smooth picture is not obtained.
In the case of a picture having a thin diagonal line, as shown in FIG. 36, for example, a difference value between two pixels 81 and 82 in a vertical direction, a difference value between two pixels 83 and 84 in one diagonal direction, and a difference value between two pixels 85 and 86 in the other diagonal direction, centered at an interpolation pixel IN, are equal to one another. Therefore, the direction in which there is a high correlation may, in some cases, be erroneously judged.
In a pixel interpolation circuit disclosed in Japanese Patent Application Laid-Open No. 2642261 (JP-A-5-68240), a method of extracting a total of six peripheral pixels, i.e., respective three pixels on scanning lines above and below an interpolation pixel, judging which of a vertical direction, a diagonally rightward direction and a diagonally leftward direction is the direction in which there is a high correlation using an interpolation table previously produced, and calculating the value of the interpolation pixel in the direction in which there is the highest correlation is employed.
When the value of the interpolation pixel is calculated using the pixel interpolation circuit disclosed in Japanese Patent Application Laid-open No. 2642261, however, noise may, in some cases, be produced.
In the case of a picture having an edge, as shown in FIG. 37(a), for example, the pixel interpolation circuit disclosed in Japanese Patent Application Laid-open No. 2642261 judges the direction in which there is a high correlation using an interpolation table previously produced with respect to six pixels A to F on scanning lines above an interpolation pixel IN. In this case, the pixel interpolation circuit judges that there is the highest correlation in the diagonally leftward direction, and calculates the value of the interpolation pixel IN using the pixel A in a direction diagonally upward toward the left and the pixel F in a direction diagonally downward toward the right, as shown in FIG. 37(b). Since the pixel A in a direction diagonally upward toward the left is “white”, and the pixel F in a direction diagonally downward toward the right is “white”, the interpolation pixel IN is also calculated as “white”. In the case of the picture shown in FIG. 37(a), however, the interpolation pixel IN should be “black”. As a result, the interpolation pixel IN becomes noise.